[booru] allow multiple 'url' keys (#5859)

docs/configuration.rst

@@ -4577,14 +4577,18 @@ Description
 extractor.[booru].url
 ---------------------
 Type
-    ``string``
+    * ``string``
+    * ``list`` of ``strings``
 Default
     ``"file_url"``
 Example
-    ``"preview_url"``
+    * ``"preview_url"``
+    * ``["sample_url", "preview_url", "file_url"}``
 Description
     Alternate field name to retrieve download URLs from.
 
+    When multiple names are given, download the first available one.
+
 
 extractor.[manga-extractor].chapter-reverse
 -------------------------------------------

gallery_dl/extractor/booru.py

@@ -29,7 +29,11 @@ class BooruExtractor(BaseExtractor):
 
         url_key = self.config("url")
         if url_key:
-            self._file_url = operator.itemgetter(url_key)
+            if isinstance(url_key, (list, tuple)):
+                self._file_url = self._file_url_list
+                self._file_url_keys = url_key
+            else:
+                self._file_url = operator.itemgetter(url_key)
 
         for post in self.posts():
             try:
@@ -74,6 +78,11 @@ class BooruExtractor(BaseExtractor):
 
     _file_url = operator.itemgetter("file_url")
 
+    def _file_url_list(self, post):
+        urls = (post[key] for key in self._file_url_keys if post.get(key))
+        post["_fallback"] = it = iter(urls)
+        return next(it)
+
     def _prepare(self, post):
         """Prepare a 'post's metadata"""